tree-optimization/111000 - restrict invariant motion of shifts

The following restricts moving variable shifts to when they are
always executed in the loop as we currently do not have an efficient
way to rewrite them to something that is unconditionally
well-defined and value range analysis will otherwise compute
invalid ranges for the shift operand.

	PR tree-optimization/111000
	* stor-layout.h (element_precision): Move ..
	* tree.h (element_precision): .. here.
	* tree-ssa-loop-im.cc (movement_possibility_1): Restrict
	motion of shifts and rotates.

	* gcc.dg/torture/pr111000.c: New testcase.